Construction of the second phase of the BART Silicon Valley Extension — the six-mile leg running from San Jose’s West Portal through downtown toward Berryessa — remains “on time and on budget,” according to the Santa Clara Valley Transportation Authority (VTA).
Single-Bore Approach Gains Ground
The project uses a single-bore tunnel design: a massive cylindrical structure large enough to house tracks, stations, and infrastructure within a single bore. A custom-built tunnel-boring machine — described by project staff as “a sideways steel can” — will dig the five-mile tunnel starting at the West Portal site.
As of December 2025, more than 90 % of the structural “walls” required to begin tunnelling have been poured, reflecting roughly 45,000 cubic yards of concrete, or about 4,500 concrete-truck loads. The timeline still targets a tunnelling start around 2028.
Scope: Rail, Stations — and Future Development
Once completed, Phase II will deliver six miles of new heavy-rail track and four new stations — marking the largest public works undertaking in Santa Clara County’s history.
But the project reaches beyond transit: VTA estimates as much as 45 million square feet of housing, retail, and commercial development will follow the rail — potentially reshaping neighborhoods and land-use patterns across San Jose and Santa Clara.
On Budget — For Now
VTA remains confident that the project stays within its cost window of US$12.1–12.5 billion, thanks in large part to the single-bore strategy and a commitment to maintain progress. Every month of delay, VTA warns, could add roughly US$20–30 million to the bill.
That said — while Phase II’s start is secure — major surface-level work (stations, yards, above-ground tracks) has yet to ramp up. As the digging proceeds, these subsequent components will bear close watching when it comes to cost control and schedule discipline.
What This Means for Tunnel Builders & Engineers
For professionals in tunnelling, civil engineering, and urban infrastructure, the BART Silicon Valley Phase II project offers several lessons:
- The viability of single-bore large-diameter tunnels for mixed-use subway construction in complex urban environments;
- The importance of pre-tunnel structural work (e.g., wall pours, portal preparation) to ensure safety and schedule integrity;
- The potential for transit-oriented development (TOD) to dramatically increase project payoff, extending value far beyond the rail line itself.
With tunnelling expected to begin in 2028 and riders projected to arrive by 2037, Phase II of the BART Silicon Valley Extension remains one of the most ambitious—and closely watched—tunnel projects in the United States. For further information please visit the tunnelbuilder archive, 50/25.
